Solid state diodes are extensively used in many electronic applications, especially in connection with vehicle electrical systems. The largest use in vehicles is as rectifiers in the vehicle alternator. Frequently, the diodes are constructed using a silicon diode chip which has one face soldered to a nickel coated metal base and with a nickel coated copper lead attached to the other face of the chip. The chip is covered with an encapsulation of some type. The base is usually designed to be press-fitted, clamped or soldered into a rectifier bridge assembly mounted in an end bell of the alternator. Many millions of alternator diodes have been made in various forms each year for decades.
FIG. 1 shows a simplified side and partially cut-away cross-sectional view of one such, prior art, press-fit diode 10, useful for vehicle applications. Diode 10 has cylindrical base 12 with cavity 14. Lower face 11 of base 12 is conveniently used for heat removal. Lower face 13 of semiconductor die 16 is mounted on cavity bottom 15 by solder 18. Die 16 contains at least one PN junction. Lead 20 has flattened portion 22 attached to upper face 17 of die 16 by solder 24. Such leads are referred to in the art as "nail-head" leads. Edge 26 of die 16 is covered and the remaining space in cavity 14 is filled with encapsulant 30 of, for example, silicone rubber. Stress relief bend 32 is frequently provided in lead 20.
While diode 10 is simple and comparatively inexpensive to manufacture, and has been widely used for years, it suffers from a number of limitations well known in the art. For example, when diode 10 is temperature cycled (e.g., by applying and removing power), there is differential expansion and contraction of the various parts. This causes fatigue of solder joints 18, 24 which may eventually crack or separate so that the diode becomes electrically inoperative or of high resistance.
Another problem has to do with shrinkage of encapsulant 30. If adhesion of encapsulant 30 to base 12 is less than perfect then encapsulant 30 may separate from base 12 as it cures or cools. When combined with solder fatigue failure, this can lead to mechanical failure of the diode.
A number of attempts have been made to overcome these and other problems associated with such prior art devices. FIGS. 2 shows a simplified side and partially cut-away cross-sectional view of, prior art, press-fit diode 34, similar to that described in U.S. Pat. Nos. 3,743,896-Welske et al., and 3,717,523-Dunsche, which are incorporated herein by reference. Device 34 has base 36 with cavity 38. Die 16 is mounted to bottom 15 of cavity 38 by solder 18 and to flattened nail head 22 of lead 20 by solder 24, in substantially the same manner as in FIG. 1, and passivation 28 (e.g., silicone rubber) is provided on die edge 26. In diode 34, nail head 22 of lead 20 is forced against die 16 by annular spring washer 39, which in turn is held in a compressed state by internal annular lid 40 of electrically insulating material. Annular lid 40 is held in place in cavity 38 by inwardly bent wall portion 41 of base 36, which is bent over after die 16, solder 18, 24, passivation 28, spring 39 and lid 40 have been installed in cavity 38 and spring 39 compressed. Outer cover or encapsulation 42 (e.g., epoxy resin) is then provided over inner lid 40 and wall portion 41 of base 36.
An advantage of the arrangement of FIG. 2 is that spring 39, lid 40 and wall portion 41 hold lead 20, die 16 and base 36 together even if solder 18, 24 fails due to thermal cycling fatigue. Thus, the diode may continue to operate electrically even after it has been thermally cycled to the point of solder failure. While this is a great advantage from a reliability point of view, the structure of FlG. 2 is comparatively very complex and is prohibitively costly in many applications.
Another approach that has been tried in the prior art for improving the thermal or power cycling performance of axial lead diodes for alternators and the like, is illustrated in FIG. 3. This device uses a base with a pedestal rather than a cavity. Diode 43 has metal base 44 with external heat transfer surface 11' and pedestal 45. The flat upper surface of pedestal 45 forms die attachment region 15' analogous to die bonding region 15 of FIGS. 1-2. Lower surface 13' of die 16' is attached to die attachment region 15' of pedestal 45 by solder 18' and nail head 22' of lead 20' is attached to upper surface 17' of die 16' by solder 24' in the same manner as for FIGS. 1-2.
Device 43 has annular plastic sleeve 46 attached to base 44 near its periphery and substantially below the elevation of die attachment region 15' on pedestal 45. Annular protrusion 47 is provided extending from pedestal 45 toward sleeve 46. Passivant 48 is provided on die edge 26' and encapsulation plastic 49 is provided covering nail head 22' and filling the remaining space between sleeve 46 and pedestal 45, protrusion 47 and passivant 48. While the device of FIG. 3 provides improved resistance to thermal fatigue as compared to the structure of FIG. 1, it does not eliminate the problem. Further, the device of FIG. 3 requires additional piece parts and, other things being equal, has a greater thermal impedance between die 16 and heat transfer face 11'.
Despite their long standing use and the extensive engineering development that has gone into the design and manufacture of such diodes, a number of problems and deficiencies remain. For example, the lead soldered to the chip face often fails under moderate tension, the encapsulant cracks or fractures during lead bending and vibration, the solder joints between the chip and the base and/or between the chip and the lead fatigue and fail under temperature and power cycling, and/or the finished diode is unduly expensive. None of the prior art approaches have overcome these and other deficiencies well known in the art. Thus, a need continues to exist for improved diode structures and methods of fabrication which have adequate lead pull strength and improved resistance to catastrophic thermal cycling failure, but which are still of low cost.